The present invention relates generally to material mixing devices. More particularly, the present invention is concerned with a mixing system adapted to control the quantity and viscosity of a slurry mixture comprised of gypsum and water. U.S. Class 366, subclass 2 is believed most relevant to classification.
Slurry mixtures may be formed from a variety of substances. One example is gypsum. Gypsum may be mixed in some form of tub or reservoir with a continuous mixing agitator blade. The slurry output is then pumped toward a remote location. To facilitate pumping the consistency of the mixture must be carefully controlled. Calcined gypsum will quickly hydrate when mixed with water. The ultimate strength of the gypsum-water mix depends primarily on the ratio of gypsum to water. The higher the ratio of gypsum to water, the greater the strength of the resultant hardened gypsum.
Since gypsum and water start hydrating immediately after they are mixed, the "setting interval" is of such short duration that the mix must be blended and pumped within a matter of ten to fifteen minutes. This factor has necessitated widespread employment of continuous mixing and pumping operations. The mixing vat employed for mixing is usually of cylindrical dimensions in the form of a tub. A water ring fabricated upon the upper periphery of the tub uniformly introduces water into the interior, whereupon a lower agitator blade mixes water with gypsum. Gypsum may be dumped directly into the mixing tub, or it may be conveyed by augers or the like. The consistency of the mix depends upon the ratio of the amount of water entering the tub and the rate at which gypsum is fed into the mixing tub.
In known prior art mixing operations the ratio of solid or aggregate material (i.e. gypsum) to liquid (i.e. water) may vary undesirably as product bags are periodically emptied into the mixing tub by workmen.
Basic prior art devices related to mixing are described in U.S. Pat. No. 1,762,081 issued to G. Schleicher June 3, 1930; U.S. Pat. No. 3,820,760, issued to Milik on June 28, 1974; U.S. Pat. No. 2,689,114 issued to Hammond on Sept. 14, 1954; and, U.S. Pat. No. 419,200 issued to Kahlengerg on Jan. 14, 1980. Mixing devices particularly adapted for handling concrete or the like are disclosed in U.S. Pat. No. 3,188,059, issued to W. A. Strong, a co-inventor hereof, on June 8, 1965; and U.S. Pat. No. 3,243,163 issued to G. Brown Mar. 29, 1966. The latter Brown patent discloses a variety of systems for mixing a plurality of varying ingredients as desired. An expansion mixer system is described by Frost in U.S. Pat. No. 3,986,707, issued Oct. 19, 1976.
In the prior art it has also been suggested to regulate viscosity of a slurry mixture with a derived signal employed to control the introduction of water. The control signal is typically derived down stream from the slurry output pump, and a variety of means may be employed to generate the signal. In U.S. Pat. No. 2,896,656, issued to Allen on July 28, 1959 a branch pipe is employed to magnetically sense slurry viscosity. A control circuit generates a signal for varying mixing water introduced via an "upstream" water control valve. Coats, issued U.S. Pat. No. 3,195,866 on July 20, 1965, monitors agitator blade motor current to derive a control signal for varying water inputted into the mixing vat. C. D. Huntington, issued U.S. Pat. No. 3,129,928 on Apr. 21, 1964, discloses a system for sensing the electrical conductivity of a slurry mixture, and for generating a water control signal in response to varying slurry conductivity.
U.S. Pat. No. 3,300,193, issued to C. O. Badgett on Jan. 24, 1967, discloses a blending system into which a plurality of aggregates are continuously fed. A separate rate control system is employed in conjunction with each of the aggregate feeders for controlling delivery of the incoming raw materials. Radiation is employed to measure the rate of travel of constituent aggregates, whereby to generate a control signal which regulates the conveyed mass of incoming raw materials to maintain a uniform product blend.